In German Patent 26 42 019 corresponding to U.S. Pat. No. 4,139,867 a method of reproducing video signals from a tape in individual tracks is described which permits slow-motion and rapid-scanning reproduction in which the tracks running obliquely to the length direction of the tape are crossed during reproduction by the magnetic heads. To put together a complete television picture only those time sections of the reproduced signals are used which are sufficient to satisfy prescribed requirements. These selected sections are written into an intermediate memory and they are read out in a manner conforming to the appropriate television standard. The recognition of time sections of a quality worth storing is done by recognition of the signal content or by evaluation of the envelope curve of the carrier frequency recorded signals. In this system the individual lines of the television picture can at the tone of recording be provided already with addresses for purposes of storing in the intermediate memory.
Another method of reproducing video signals stored on a magnetic tape at a reproduction speed different from the recording speed is known from German Patent 27 25 365 corresponding to U.S. Pat No. 4,210,938 in which the line addresses for the intermediate storing are first provisionally established in reproduction by evaluation of the instantaneous position of the head wheel and of the tape, in each case relative to a reference position. The method assumes that the magnetization regions for each line of the television picture is always recorded in exactly the same position relative to its surrounding on the magnetic tape and also is picked up by the head wheel
Finally, a system is known from German Patent 28 41 728 corresponding to U.S. Pat. No. 4,293,879 in which the line addresses are to be derived exclusively from the reproduced video signal. For that purpose the horizontal and vertical frequency synchronizing pulses are separated from the composition video signal obtained from the tape and also the pulse modulation arising from the passage of the magnetic head from one track to the neighboring track. These are used after appropriate pulse shaping for establishing the line addresses by counting.
In a known method of magnetic storing of digitally coded television signals the data are transmitted serially by blocks after scanning of the analog values, quantization and coding with addition of identification and check values. Because of the large transmission capability necessary, in the known magnetic tape equipment for recording and/or reproducing of digitally coded television signals, recording is provided in four parallel channels with correspondingly narrower transmission bandwidths. For recording and reproduction four magnetic heads are disposed around the circumference of a head wheel at equal spacings. The envelopment angle of the tape around the head wheel is 258.degree.. The four magnetic heads contact one after the other with the magnetic tape and write on the magnetic tape in respective tracks running obliquely to the length direction of the tape. These track contain in each case two track segments, each belonging to different "sectors" of video data. These two segments are separated from each other in the region of the longitudinal axis bisecting the tape by a number of short track sections that contain different sound information. Each sector track segment contains video data differing from that of the other sectors.
Precautions are necessary in reproduction to ensure that each of the four magnetic heads read the date which were recorded by it or by an identical magnetic head of another apparatus constituted according to the same design. The particular arrangement of the track sections with video data on the magnetic tape makes necessary the switching over of the magnetic heads before and after the sectors provided for the video data from one video data channel to another of a particular video data channel. This takes place in recording in accordance with a predetermined pattern which repeats itself after each revolution of the head wheel. Further details of that system and method are to be found in the publication "Standard for Recording Digital Television Signals on Magnetic Tape in Cassettes" of the European Broadcasting Union, Tech 5252-E and also in the periodical "Fernseh- und Kinotechnik" 1987, Heft (Issue) 1/2, pages 15 to 22.
In reproduction measures must be carries out to assure that the video data subdivided over four channels are correctly put back together. For that purpose it is necessary to recognize which magnetic head reads data for which channel and from which sector. Very many and various switching schemes arise, however, when the reproduction of the recorded data is not done, as above assumed, at the same speed as recording, but at a speed different from the recording speed, for slow-motion examination of the picture or to speed up the picture or to carry out a search with visible pictures. In such a case, each magnetic head crosses the tape at an angle to the tracks and passes along two or more tracks in each scan in a manner dependent upon the tape transport speed, with an effect that is the same both in slow motion and in speeded up motion of the picture. Each magnetic head can accordingly read every track section with video data, but the video data read out by the rest of the magnetic heads are not independent from those of the first-mentioned head, but rather fixedly intercoupled therewith.
In known systems, each recorded block with digitally coded video data is preceded by an identification signal. Although by the identification signals it is possible to correlate simply, the allocation of the information read out to the sectors and thereby to the corresponding processing channels, there nevertheless are problems if an identification signal is misread and the subdivision of the data is spoiled by the misdirected switching. There is then no more storage space in the intermediate store for the correctly allocated data, for example under the preceding address. Besides, there is the risk of "bus collisions", against which therefore special precautions must be taken. In general, therefore, there is no possibility of checking the correctness of the switched paths for data paths which are automatically switched on the basis of identification signals.